In this region, beyond atomic number 26, energy is released by fission rather than fusion. At the far left of the curve, hydrogen fusion in stars releases energy:



and at the far right, uranium fission in atomic reactors also releases energy:



(This is only one of many ways in which the ²³⁵U nucleus can break down.)

The maximum stability of the iron nucleus is the reason why the element-building process by successive fusion reactions, outlined in Chapter 8, stops at iron. Beyond iron the fusion process is energy-requiring instead of energy-yielding. The heavier elements are built up by more indirect processes involving neutron capture.

In spite of the fact that mass is not conserved in nuclear reactions, conservation principles do apply to the total number of heavy particles (protons and neutrons) and total charge. This is implicit in the equations for the two previous nuclear reactions.